6.2
Spatial Characteristics

Spatial resolution is the resolving power of an instrument needed for
the discrimination of features and is based on detector size, focal length,
and sensor altitude. More commonly used descriptive terms for spatial
resolution are ground sample distance (GSD) and instantaneous field of
view (IFOV). The IFOV, or pixel size, is the area of terrain or ocean
covered by the field of view of a single detector. The ETM+ ground samples
at three different resolutions; 30 meters for bands 1-5, and 7, 60 meters
for band 6, and 15 meters for band 8. Figure 6.3 illustrates the ETM+IFOV for bands 1-5 and 7 relative to other sensors and a football field.
IKONOS, the recently launched Space Imaging sensor, has an IFOV of 1 meter.
The French SPOT panchromatic sensor an IFOV of 10 meters whereas the SPOT
multispectral (XS) sensor has an IFOV of 20 meters. ETM+ has an IFOV of
30 meters for bands 1-5, and 7 of 30 meters while the Indian Remote Sensing
Satellite (IRS) has an IFOV of 36.25 meters.

Figure 6.3 - ETM+ Spatial Resolution Relative to Other Sensors

A standard WRS scene covers a land area approximately 185 kilomenters
(across-track) by 180 kilometers (along-track). A more precise estimate
for actual scene size can be calculated from the
0R product image dimensions. These are listed in table 6.1

Table 6.1 Image Dimensions for a Landsat 7 0R Product

Band
Number

Resolution
(meters)

Samples
(columns)

Data Lines
(rows)

Bits per
Sample

1-5, 7

30

6,600

6000

8

6

60

3,300

3,000

8

8

15

13,200

12,000

8

It is natural to assume that one could determine a scene's spatial extent
by multiplying the rows and columns of a scene by the IFOV. This would
lead to a scene width of 198 kilometers (6600 samples * 30 meters) and
a scene length of 180 kilometers (6000 lines * 30 meters). While this
calculation applies to scene length, the scene width calculation is more
complicated due to the presence of image buffers and the staggered image
bands in the 0R product. Left and right image buffers were placed in the
0R product to accommodate a possible increase in scan line length over
the mission's life. The staggered image bands result from the
focal plane design which LPS accounts for by registering the bands
during 0R processing. The end result is an increasing amount of zero-fill
preamble according to the band order on the ground projected focal plane
array.

The detector offsets determine the amount of zero fill preamble for
each band. These are listed in Table 6.2 and can also be found in the
Calibration Parameter File. Coincident imagery for
all 8 bands starts at pixel location 247 for the 30 meter bands. One need
only to look at at the reverse scan odd detector offset for band 6 to
see that this is true. This number, 116, is actually in 60 meter IFOVs
which translates to 232 30 meter pixels. Another 14 pixels must be added
to this number to account for the seven minor frames of image data pre-empted
by time code. Coincident imagery for all 8 bands ends at pixel location
6333 for the 30 meter bands. This number is determined by looking at the
reverse even detector offset for band 8. Add to this number the value
12,626 which represents the number of band 8 pixels per line (6313 minor
frames times 2). The total, 12,666, is halved to put the ending pixel
number into 30 meter units. The number of coincident images pixels in
a scan is therefore 6087 (6333 - 247 + 1). The nominal width for a scene
is therefore 182.61 kilometers (6087 * 30 meters).